The present invention relates generally to ceramic arc discharge lamps and more particularly to a discharge lamp in which an oxygen content of the lamp fill during lamp operation is selected to provide a high lumen maintenance.
Discharge lamps produce light by ionizing a vapor fill material, such as a mixture of rare gases, metal halides, and mercury with an electric arc passing between two electrodes. The electrodes and the fill material are sealed within a translucent or transparent discharge vessel that maintains the pressure of the energized fill material and allows the emitted light to pass through it. The fill material, also known as a “dose,” emits a desired spectral energy distribution in response to being excited by the electric arc. For example, halides provide spectral energy distributions that offer a broad choice of light properties, e.g., color temperature, color rendering, and luminous efficiency.
Conventionally, the discharge vessel in a discharge lamp was formed from a vitreous material such as fused quartz, which was shaped into desired chamber geometries after being heated to a softened state. These lamps are limited in performance by the maximum wall temperature achievable in the quartz discharge vessel.
Ceramic discharge chambers were developed to operate at higher temperatures for improved color temperatures, color renderings, and luminous efficacies, while significantly reducing reactions with the fill material. One problem with such lamps is that the light output over time (typically expressed as lumen maintenance) tends to diminish due to blackening of the walls of the discharge vessel. The blackening is due to tungsten transported from the electrode to the wall.
It has been proposed to incorporate a calcium oxide or tungsten oxide oxygen dispenser in the discharge vessel, as disclosed, for example in WO 99/53522 and WO 99/53523 to Koninklijke Philips Electronics N.V. Lamps produced according to these applications may not, however, simultaneously meet acceptable lamp efficiency, color point, color stability, lumen maintenance, and reliability values for a commercial lamp.
The exemplary embodiment provides a new and improved metal halide lamp with improved lumen maintenance.